A partial reflection of light is always to be observed when light impinges onto a boundary surface, e.g. air/glass or vice versa. In the case of perpendicular incidence onto a sheet of glass, approximately 4% of the incident light is reflected at each of the two boundary surfaces. This value increases to approx. 5% for light incidence at an acute or obtuse angle. This partial reflection constitutes a considerable problem for many applications, for example in optical elements such as lenses, etc. in which as high a transmission as possible is desired.
Anti-reflection coatings which consist of thin films are commercially available. Such coatings are costly, however, the mechanical stability thereof is often unsatisfactory and the tolerance thereof with respect to the angle of incidence is low. Recently, to solve these problems, microstructures and nanostructures, which are similar to the structures of moth eyes and are therefore also termed moth-eye structures by way of illustration, were applied onto the surfaces of optical elements (Kanamori et al. (1999) OPTICS LETTERS 24 (20), 1422-1424; Toyota et al. (2001), Jpn. J. Appl. Phys. 40 (7B), 747-749). Most of these approaches are based on slow and costly application methods, such as e.g. electron beam lithography.
A simple and inexpensive method, using which moth-eye structures can be created directly on quartz glass by means of etching, is described in the German laid-open specification DE 10 2007 014 538 A1 and in the corresponding international publication WO 2008/116616 A1, as well as in Lohmüller et al., NANO LETTERS 2008, Vol. 8, No. 5, 1429-1433. The etching method disclosed therein is, however, not yet optimal to the extent that the moth-eye structures obtainable therewith are generally based on an arrangement of column-like structures. These column-like structures are inferior to the conical microstructures on a natural moth eye with respect to the anti-reflective action thereof. With the method mentioned, as a matter of principle it is barely possible to produce ideal conical structures, as in this process gold particles are used as etching masks and the gold particles are removed substantially more slowly than the quartz glass of the substrate surface. If one changes the parameters of the method described (e.g. additional oxygen in the process gas, smaller argon fraction), in order to achieve a more pronounced isotropic removal, although one obtains partially conical structures, these always have a relatively wide and deformed upper end which impairs the anti-reflective properties.
In Applied Physics Letters 93, 133109 (2008), Ching-Mei Hsu et al. describe a method for creating column and cone structures by means of the selective etching of a silicon substrate surface, previously applied SiO2 nanoparticles serving as etching mask. This method, however, does not enable the manufacture of column and cone structures which themselves consist of SiO2 or quartz glass.
It was therefore an object of the present invention to provide various substrate surfaces, including SiO2 surfaces and quartz-glass surfaces, particularly of optical elements, which have an anti-reflective arrangement of approximately ideal conical nanostructures, in a manner which is as simple, material-saving and cost-effective as possible.
This object is achieved according to the method, the substrate surface and the optical element according to the invention.